Ecommerce email subject lines: written with AI, then tested
AI writes the list in seconds. This is how you pick the two worth sending, pair them with preview text, and test them honestly.
Good ecommerce email subject lines are short enough to read on a phone, tell the truth about what is inside, and come with preview text that adds something new. Any AI tool can write ten of them in seconds. The work that moves results is choosing which two to send and testing them one variable at a time.
This guide gives you five approaches to write from, how to pair each subject line with preview text, why to keep lines under 50 characters, when an A/B test is worth running, and the exact prompt to use in Claude. You can follow the method by hand or with the free /margarita‑subject‑lines skill.
Write 8 lines across five approaches. Pair each one with preview text that finishes the thought. Cut anything over 50 characters and anything the email does not back up. Test two lines that differ in one way, and wait for enough sends before you pick a winner.
01Generators write lines. You still have to pick them.
Search for an AI subject line generator and you find dozens of free tools. They all do the same thing: you describe the email, they return a list. The list is rarely the problem. A list of ten lines tells you nothing about which one your customers will open, and some of those lines will promise things your email does not deliver.
Boomp describes a common workflow: paste the finished email into Claude or ChatGPT, ask for ten lines with preview text, then pick two to test, or just one if your list is small. That is a sound start. This guide adds what a generator skips: a reason to prefer one line over another, a rule for what preview text should do, and a check that every claim is true.
Always start from the finished email. A subject line written before the email exists tends to promise a different email.
02Five approaches to ecommerce email subject lines
Group your lines by approach, so you are choosing between ideas and not between near-identical wordings. The example lines below are illustrative: written for this guide for a made-up coffee brand, with no performance data behind them.
Open a question the email answers. Example: "The grinder setting most of us get wrong". It works when the email really pays it off. It fails when the answer is thin.
Lead with what the reader gets. Example: "15% off your next refill, through Sunday". Best for offer emails. The offer and the date must match the email exactly.
Use a number, a name or a concrete detail. Example: "3 ways to brew our dark roast at home". A clear expectation helps the right people open.
Refer to something you actually know about the reader. Example: "Roasts picked from your last order". Use it only when the email is built from their data. A first name merge tag on its own is thin personalization.
Say plainly what the email is. Example: "New: Ethiopia single origin is here". Good for launches and for people who already like the brand.
You do not need all five in every send. A sale email might get two value first lines and one direct line. A content email might lean on curiosity and specific. What matters is that the lines you test differ in approach, so the test teaches you something about your audience.
03Subject line and preview text: finish the thought
Marketing Monk calls preview text "your second subject line" and says it should complement the subject, not repeat it. Wild & Free Tools notes that preview text gives you roughly 40 to 90 extra characters, and warns against leaving it as default text such as "View this email in your browser".
Read the subject and the preview as one line, because that is how the reader sees them in the inbox. The subject opens. The preview adds the next piece of information. Illustrative pairs:
- Weak: "15% off your next refill, through Sunday" + "Save 15% on your next refill". It says the same thing twice.
- Better: same subject + "Applies to every roast, decaf included." It adds a fact the reader did not have, and it is only allowed if the email says so.
- Curiosity: "The grinder setting most of us get wrong" + "Plus the fix, with photos of each step." It promises the payoff without giving it away.
If your email platform fills preview text from the first line of the email, look at what that first line is. Often it is a view in browser link or an image's alt text. Set the preview text by hand on every send.
04Why under 50 characters on a phone
Phone inboxes show less of a subject line than desktop does, and the exact cutoff changes with the mail app, the screen size and the reader's font setting. You cannot control any of that. The safe move is to keep the whole line under 50 characters and the meaning in the first few words. That is the limit /margarita‑subject‑lines writes to.
- Put the key word first. If a line only makes sense once you reach the last three words, a phone may hide those words.
- Count characters, not words. Short words still add up.
- Skip emoji unless the brand already uses them. They can render differently across apps and take space you need.
The real check is cheap: send the email to yourself and look at it in your phone's inbox. If the line is cut or the preview starts with junk, fix it before it goes to the list.
05The honesty rule
Every claim in the subject line and preview text must appear in the email. If the subject says 15% off, the email says 15% off, on the same products, with the same end date. If the subject says back in stock, the product is in stock when the email lands.
No fake urgency. "Last chance" when it is not the last chance, "ending soon" with no end date, a timer that resets: readers notice, and you train them to ignore you on the day it is true. If there is a real deadline, state it plainly. If there is none, do not invent one.
- No ALL CAPS lines.
- No fake "RE:" or "FWD:" that pretends to be a reply.
- No emoji unless the brand already uses them.
- No stock, price or review claims the email does not contain.
The pre-send QA step checks subject honesty too. See the email QA checklist for the full list.
06The prompt
Attach the finished email draft (HTML, a screenshot or plain text) and fill the brackets. With the skill installed, the first line calls it. Without the skill, delete the first sentence: the numbered steps still work in any Claude chat, you just write the rules out yourself each time. /margarita‑subject‑lines is free and arrives by email when you join the Margarita Creative newsletter.
Use /margarita-subject-lines. Attached is the email draft. Offer: [OFFER, OR NONE]. Audience: [SEGMENT]. Send day: [DAY].
1. Write 8 subject lines under 50 characters, grouped by approach.
2. Pair each one with preview text that finishes the thought instead of repeating it.
3. Pick two for an A/B test and say what the test will teach me.
No fake urgency, no ALL CAPS, and no emoji unless the brand already uses them. Every claim must appear in the email.You get back 8 lines grouped by approach, preview text for each, and two picks for a test with a sentence on what the result would tell you. Read that last sentence closely. If it cannot name a lesson, the two lines are too similar to be worth testing.
07How to A/B test and check the output
Run a test when your list is big enough that each version reaches a real number of people. If it is small, take Boomp's advice: pick one strong line and send it. A test split across too few people mostly measures noise.
Change one variable. Two lines that differ only in approach, say value first against curiosity, teach you what your audience responds to. Two lines that differ in approach, length and emoji at once teach you nothing, because you cannot tell which change did the work. Keep the preview text in the same style for both, or test it on its own in a later send.
Write the result down next to the variable you tested. After a handful of sends you have a record of what your own list opens, which is worth more than any generic list of best subject lines.
1. Read each line in a phone inbox preview: not cut, no default preview text. 2. Check every claim appears in the email: offer, price, date, product, stock. 3. Check each pair: the preview adds something, it does not repeat. 4. Test one variable at a time and note which one. 5. Wait for enough sends before you call a winner. An early lead can flip.
Then run the email through pre-send QA. Subject lines are workflow 4 of 6 in Claude for email marketing: the 6 workflows, and if you build emails with the Margarita Email Designer, it already drafts subjects A and B that you can check with the same rules.
08Quick answers
What is the best length for an ecommerce email subject line?
Under 50 characters is a safe working limit, because phone inboxes cut longer lines and the cutoff varies by app and screen. Put the key word first so the meaning survives even if the end is hidden.
What should preview text say?
It should finish the thought the subject line starts and add a new detail, not repeat the subject. Never leave it as default text like "View this email in your browser".
Can I just use an AI subject line generator?
Yes, for drafts. Paste the finished email into Claude, ask for lines grouped by approach with preview text, then choose and check them yourself. A generator does not know which claims your email can back up.
How many subject lines should I A/B test?
Two, that differ in one variable. If your list is small, send one strong line and skip the test.
Do I need the /margarita‑subject‑lines skill?
No. The prompt and checks in this guide work in any Claude chat. The skill builds in the grouping by approach, the 50 character limit, preview text and an A/B test pick, so you do not have to spell them out each time.